The present invention relates to a solenoid valve which is arranged so as to form a switch by a valve and the associated valve seat.
In the prior art, for obtaining an electric signal indicating the open/close state of a valve unit, it is well known to use a valve unit in which an ON-OFF switch is constituted by a valve and the associated valve seat. Such a solenoid valve unit having such a switch is needed, for example, for constituting a valve unit driving circuit in which the driving pulse applied to the valve unit is corrected in response to the timing of opening/closing of the valve unit in order to make the open/close timing of the valve unit coincide with a target timing. It is also needed for constituting a fuel injection valve which is capable of producing an electric signal indicating the injection timing of fuel.
As such a valve unit, there is disclosed for example in U.S. Pat. No. 4,111,178 (corresponding to DE-OS No. 2748447) a fuel injection valve in which a mechanical switch is constituted by a needle valve and a nozzle body in order to obtain an electric signal indicating the timing of the beginning of fuel injection and the timing of the end of fuel injection in response to the movement of the needle valve. In the disclosed fuel injection valve, a nozzle body and a needle valve smoothly moving in the guide hole of the nozzle body are formed of an electrically conductive material and the outer surface of the needle valve is covered with a ceramic insulation layer of a thickness between approximately 0.2 .mu.m and 0.3 .mu.m, or an insulation layer formed by the sputtering of aluminum oxide.
However, since the insulation layer of the conventional switch incorporated in the solenoid valve is formed only on the sliding surface of the valve member, it has the following disadvantages. Namely, although it is required for such a switch to maintain an open state during the open state of the valve, just after the opening of the valve or just before the closing of the valve, there is a possibility of the occurrence of an undesired electrical conducting state between the valve member and a member for determining the maximum stroke of the valve member for the opening state of the solenoid valve, whereby a noise signal is superposed on the desired signal derived from the switch. Consequently, in the case where the conventional switch is used, it is sometimes required to remove such an undesired signal by means of a complex signal processing circuit.
Furthermore, when the insulation layer or film is formed on the outer periphery of the valve member, the insulation layer is liable to peel off and the machining required for assuring a perfect fit between the valve member and the guide hole becomes difficult, so that the manufacturing cost is increased.